A247484 - OEIS

A247484

a(1)=8; for n>=1, a(n+1) is the smallest palindromic 3-almost prime with a(n) as a central substring.

%I #15 Dec 02 2014 20:56:36

%S 8,282,52825,4528254,145282541,2214528254122,122145282541221,

%T 51221452825412215,7512214528254122157,15751221452825412215751,

%U 7157512214528254122157517,371575122145282541221575173,93715751221452825412215751739,3937157512214528254122157517393

%N a(1)=8; for n>=1, a(n+1) is the smallest palindromic 3-almost prime with a(n) as a central substring.

%C The 3-almost primes are the numbers that are the product of exactly three (not necessarily distinct) primes.

%e a(1) = 8 = 2*2*2;

%e a(2) = 282 = 2*3*47;

%e a(3) = 52825 = 5*5*2113;

%e a(4) = 4528254 = 2*3*754709.

%t d[n_]:=IntegerDigits[n];t = {x = 8}; Do[i = 1; While[!PrimeOmega[y = FromDigits[Flatten[{z = d[i], d[x], Reverse[z]}]]]==3, i++]; AppendTo[t, x = y], {n, 13}]; t

%Y Cf. A014612, A247483.

%K nonn,base

%O 1,1

%A _Michel Lagneau_, Dec 01 2014